Switching of digital signals from a service (working) channel to an alternate (protect) channel, and vice versa, in a communications system can cause a "hit" to the traffic. In other words, the payload can be corrupted during this switch from the working channel to the protect channel. This corruption occurs because the protect channel and the working channel have different signal payload pointer values, and a processor circuit must recognize the different pointer values and align its counter. During this time the traffic can be corrupted.
Hit-less (i.e., error-less) switching systems (for example, for digital radio) are known. In these systems, signals carried on a working channel are switched to a protect channel upon the detection of a predetermined threshold number of errors at the receiver. These prior switching systems accomplish this switching by compensating for the different transmission delays between the working and protect channels by incorporating into each channel a delay build-out equal to the maximum differential delay between the two channels. Furthermore, an additional variable delay can be controllably added to the protect channel. By varying the length of this variable delay, a delay can be added to the protect channel such that the total delay of the protect channel is equal to the total delay of the working channel. The total delay of the working channel is equal to the inherent delay in the working channel plus the delay build-out. Once the delay between the channels has been equalized, the signal can be hit-lessly switched from the working channel to the protect channel.
Such an error-less switching system, however, can only be used when the bit stream transported on the protect channel is identical to the bit stream transported on the working channel. Such a system cannot be used when employing a signal format (for example, the SONET format) in which a payload (i.e., a predetermined unit of data) and a marker indicating the location of the payload can float within each frame and where the bit streams arriving at the receiver from the working and protect channels may not be identical. Furthermore, the overhead bytes of the signal carried on the working channel may be different than the overhead bytes of the signal carried on the protect channel, even if the starting location of the payload and corresponding frames is the same. Signals carried on a SONET network may, for example, need to be re-routed from the working channel to an alternate protect channel which may pass through an intermediate central location. To keep the payload synchronous with the SONET network, the payload may be shifted within the frame. The signals that arrive at the receiving end on the working and protect channels could then possibly be different.
U.S. Pat. No. 5,051,979 claims to teach a method for achieving hit-less switching between SONET signals. In this method, each STS-1 signal is frame-aligned by an individual delay buffer and sent to an individual pointer processor. Inside the pointer processor, each signal's payload data is extracted and inserted into a new frame, along with a new pointer value. At the output of each pointer processor, the new pointer values are sent to a pointer justification and controller circuit. One of the pointer processors is designated the master and the other pointer processor is designated the slave. The pointer justification and controller circuit monitors the pointer value generated by the master pointer processor and, based on the master pointer value, sends justification control information to the slave pointer processor. Thus, at the output of the two pointer processors, the pointer values match. Subsequently, each signal is sent to a 2:1 multiplexer which selects between the two STS-1 channels. Because the signals are both frame- and payload-aligned, a simple 2:1 selection can cause a switch from the working channel to the protect channel and back.
While this method accomplishes hit-less switching in switching systems employing a signal format such as the SONET format, the solution is complex and costly because an initial frame alignment must be performed. Furthermore, this approach requires the generation of two distinct SONET frames before performing the hit-less selection. The circuitry required to implement this solution is complex, requiring a separate pointer processor to regenerate a new SONET frame for each of the two channels before the protection switch can occur.